In a digital picture (or image) communication system or a digital picture communication service, the technique of encoding the picture information to reduce the information volume to effect transmission and storage is used in transmitting and storing the picture information. As an encoding system for moving picture, internationally standardized by ITU-T (International Telecommunication Union), there are known an encoding system prescribed by H. 261 Recommendations providing for standardization of transmitted pictures of visual telephone and video conference and H. 263 Recommendations providing for standardization of pictures transmitted over a low bitrate network, such as PHS (Personal Handy-Phone System).
In the encoding system for moving pictures, internationally standardized by ISO (International Organization for Standardization), there is known MPEG (Motion Picture Experts Group) 1, as an encoding system for stored video images, while there are known MPEG-2 and MPEG4 as a universal encoding system and a low bit rate encoding system, respectively.
The respective encoding systems used for moving picture compression are merely analogous to one another to the extent that these systems use the DCT (Discrete Cosine Transform), motion compensation prediction and Huffman codes in common, which constitutes only a part. However, bitstreams obtained on actual encoding differ from one system to another.
So, if it is desired to interconnect a system employing a certain picture encoding system to another encoding system employing another encoding system, picture codes, once formed, need to be decoded into picture signals, which then need to be re-encoded as an input picture.
For example, if the H. 261 bitstream is to be converted into that of H. 263 or MPEG4, not having the in-loop filtering function, such as one provided in H. 261, it is necessary to decode the encoded bitstream into an input picture and to re-encode the resulting picture signals.
On the other hand, if it is desired to convert a bitstream of H. 263 or MPEG4 into that of H. 261, it is necessary to decode the bitstream into a picture and to re-encode the resulting picture. This is because the maximum value of the motion vector of H. 263 or MPEG4 is larger than that of the motion vector of H. 261, while the motion vector of H. 261 merely has the vector merely of an integer precision.
That is, for converting a picture encoding system into another appreciably different picture encoding system, it is necessary for transforming a bitstream to interconnect a decoder to an encoder, first to revert the input bitstream transiently into picture signals by a decoder, and then to re-encode the picture signals as an encoder input signal.
For moving picture data, encoded in accordance with a certain encoding system, there is known an encoding rate converting system, as a method for realizing rate conversion between an input bitstream and an output bitstream. This rate conversion is carried out when the bandwidth of a transmission channel for the input bitstream differs from that of transmission channel for an output bitstream.
FIG. 10 shows an embodiment of a structure of a conventional picture encoding system converting device. Referring to FIG. 10, this picture encoding system converting device includes a buffer for storing a bitstream output from outside, a decoder 6 for decoding picture codes output from the buffer 5, an encoder 7 for encoding picture signals output from the decoder 6 and another buffer 8 for storing picture codes output from the encoder 7 to output the picture codes to outside.
The encoder 7 monitors the storage volume (occupied volume) in the buffer 8. This stored volume in the buffer is utilized for controlling the volume of the generated codes during encoding by the encoder 7.
As this sort of the picture encoding system conversion device, there is proposed in e.g., JP Patent Kokai JP-A-7-107461 a picture encoding system conversion device adapted for storing the hysteresis information of encoding parameters, such as motion vector or quantization step size etc., used in decoding, and referencing these encoding parameters to determine the encoding parameters to effect re-encoding.
In e.g., JP Patent Kokai JP-A-7-288804, there is proposed, as a re-encoding device for picture signals in which it is possible to prevent picture quality deterioration resulting from re-encoding and to freely select the picture quality on re-encoding, a configuration in which the number of quantization bits is set in addition to the prediction mode, motion vector and the quantization step size, as encoding parameters obtained on decoding an input bitstream, to enable re-encoding with an optional data amount.
In e.g., JP Patent Kokai JP-A-8-111870, there are also proposed, as a method and apparatus for re-encoding the picture information in such a manner as to assure optimum picture quality even for a picture having encoding hysteresis, a method and apparatus in which re-encoding is achieved using the prediction mode, motion vector, quantization step size or periods or phase of picture types, as encoding parameters obtained on decoding the input bitstream.
In e.g., JP Patent Kokai JP-A-10-32830, there is proposed a device for re-encoding picture signals in which the quantization step size acquired on decoding an input bitstream is used to determine the quantization step size of an encoder to realize the re-encoding.
In e.g., JP Patent Kokai JP-A-10-336672, there is proposed, in an encoding system conversion device in which, in re-encoding encoded picture data in accordance with a different encoding system, the processing volume is diminished without deteriorating the motion vector detection accuracy, a configuration in which the motion vector obtained in decoding encoded picture data is stored and scaled depending on the conversion scale of the picture size, or is converted in quantity depending on the number of frames, the so scaled or converted motion vectors are provided as candidates and one of these candidates is used to effect re-encoding.
In e.g., JP Patent Kokai JP-A-11-285002, there is proposed a moving picture encoding device in which quantization control is performed on a bitstream obtained on encoding a moving picture so that the bitrate of a bitstream acquired on re-encoding will fall within a preset range.
As another example of the encoding rate conversion device, there is proposed in e.g., JP Patent Kokai JP-A-8-251587 a configuration of a rate conversion device for encoded picture data having a performance comparable to the transcoding using means simpler than the transcoding. In the above-described encoding rate converting device, disclosed in the JP Patent Kokai JP-A-8-251587, there is proposed a configuration in which inverse-quantized encoded data is re-quantized to control the rate control of the quantization level. However, this conventional encoded picture data rate conversion device, described in the JP Patent Kokai JP-A-8-251587, which decodes the received bitstream and uses the resulting encoding parameters to improve the picture quality on re-encoding, fails to take into account the delay time produced on effecting conversion of the encoding system.